This invention relates to an etching method, etching apparatus and analyzing method. More particularly, the invention relates to an etching method, etching apparatus and analyzing method used for detecting impurities contained in a semiconductor substrate, semiconductor thin film, and so forth, by chemical analysis with a high sensitivity.
Sodium (Na), iron (Fe) and other various metal impurities existing in semiconductor thin films and semiconductor substrates invite deterioration in insulation-to-voltage property of oxide films and production of crystallographic defects therein, and therefore greatly affect the characteristics of semiconductor devices. Taking it into consideration, device manufacturing processes usually include a step of removing metal impurities from device-making regions by washing or gettering. Recently, however, the industry is under rapid progress toward higher integration, miniaturization and higher performance of semiconductor devices. Along this tendency, allowance to metal impurity concentration is getting lower and lower, and much higher sensitivity and reliability are required in impurity analysis for controlling contamination. Simultaneously, there is a strong demand for a method not only capable of conventional surface analysis but also capable of analyzing impurities contained in bulk of semiconductor thin films and substrate surface layers for actually making devices.
As one of methods for analysis of impurities contained in bulk of substrates, SIMS (secondary ion mass spectroscopy) is now being used widely. This method has large advantages, e.g., high resolution as high as 1 .mu.m or less, ability of analysis in a depth direction, and availability for analyzing various substances from hydrogen (H) to uranium (U). However, its detectable limit for Fe, representative contaminant, is as high as 10.sup.15 atoms per cm.sup.3, which is insufficient as the sensitivity required today for appreciating contamination.
On the other hand, electric methods such as DLTS (deep level transient spectroscopy), .mu.-PCD (photoconductivity decay) or SPV (surfacephoto voltage) have very high sensitivities, as their detectable limit as high as 10.sup.10 atoms per cm.sup.3 represents. However, there is a large restriction in measurable substrates and elements, and their application is limited to certain purposes.
Chemical analysis, which etches a substrate and analyzing impurities in the etching solution, is somewhat complex and requires a sill to conduct it. However, it is greatly advantageous in promising a high sensitivity with its detectable limit being as high as 10.sup.11 to 10.sup.14 atoms per cm.sup.3 and in having no restriction in detectable substrates and elements. Representative one of such methods is disclosed by Takenaka et al. in "Analytic Chemistry" (Vol. 43, pp. 173-176 (1994)). This is a step etching method which etches a substrate fixed in a Tefion-made decomposition vessel by injecting a HF/HNO.sub.3 mixed solution on the substrate surface. There is another method disclosed by M. Shabani et al. in Proceedings of 41st Joint Symposium of Japan Society of Applied Physics No. 2, p 598. This is a drop etching method in which a HF/HNO.sub.3 mixed solution is sandwiched between a Teflon plate and a silicon substrate, and the surface of the silicon substrate is etched.
All of these techniques are characterized in dissolving silicon (Si) by exothermic reaction expressed by three formulas shown below, then changing the etching depth by changing the HF concentration in the mixed solution (the HF concentration and the etching depth are proportional), and thereby enabling analysis in the depth direction. EQU Si+2HNO.sub.3.fwdarw.SiO.sub.2 2N.sub.2 O.sub.3.uparw.+H.sub.2 O EQU SiO.sub.2 +4HF.fwdarw.SiF.sub.4.uparw. EQU SiO.sub.2 +2HF.fwdarw.H.sub.2 SiF.sub.6.uparw.
Additionally, by further concentrating the etching solution, these techniques can further increase the sensitivity.
In order to lower the detectable limit and increase the sensitivity in chemical analysis, it is necessary to reduce contamination from outside and increase the sensitivity of an analyzer. Since the requirement to analyzer is not easy to realize, current development is centrally directed toward techniques for reducing external contamination. "External contamination" herein pertains to the following three modes of contamination.
(1) contamination from reagents used (contamination contained in reagents) PA1 (2) contamination from environments of analysis (clean room and booth for analysis) PA1 (3) contamination from tools for analysis Among these modes of contamination, (1) contamination from reagents is currently the most important issue. If the amount of impurities contained in a reagent, the back ground level of impurities becomes high and the detection sensitivity decreases. To prevent this problem, a reagent with a super-high purity must be used for analysis. However, as the used amount increases, the amount of contamination also increases so much, and the back ground level of impurities increases accordingly. Therefore, it is important to minimize the used amount of reagents in impurity analysis. PA1 heating or cooling said surface of said object, said heating or cooling being conducted unevenly on a space basis or a time basis in order to make etching of said object progress uniformly over the entirety of said surface. PA1 The heating or cooling can be conducted locally on said surface of said object. PA1 providing the plate member with a permeable means for permitting gases to pass through, and externally releasing gases generated by etching through the permeable means. PA1 controlling the temperature of the surface of the object to become higher in a final stage of etching than in an initial stage of the etching. PA1 heating or cooling means for locally heating or cooling the surface of the object to have etching of the object progress uniformly over the entirety of the surface. PA1 said plate member having a permeable means for externally releasing gases generated by etching. PA1 means for controlling the temperature of the surface of the object to become higher in a final stage of etching than in an initial stage of the etching.
Regarding (2) contamination from environments, even when works are conducted in a clean booth of the class 10 or higher installed in a clean room having a cleanness of the current class 100 or higher, there is still a possibility of contamination, depending on elements. Therefore, works of analysis (decomposition and concentra-tion) must be finished in minimum time. Regarding tools for analysis, by using Teflon as the material and washing it well before analysis, reduction of contamination to zero has been accomplished. In relation to the above-made explanation, advantages and problems of step etching and drop etching are summarized in Table 1.
TABLE 1 Advantages and Problems of Step Etching and Drop Etching Step Etching Drop Etching Etching depth 0.01.about.10 .mu.m 0.1.about.1 .mu.m (6 inches) Amount of 10 ml 1 ml Chemicals Decomposition 5 minutes 5 minutes Time Advantages .cndot.Etching to a deep re- .cndot.No contamination by gion is possible (etching chemicals (using very under over-supply of small amount of chemi- HNO.sub.3) cals) .cndot.No contamination from environments (time for concentration is short (&lt;1 hr)) Problems .cndot.Contamination by rea- .cndot.Maximum etching depth gents (relatively large is 1 .mu.m (because of short- amount of reagents is age of HNO.sub.3, reaction does used) not progress beyond 1 .mu.m) .cndot.Contamination from .cndot.Uneven etching due to environments (due to a existence of a plurality of long time for concentra- mountains on the etching tion) surface (FIG. 8) .cndot.Over-etching in form of an inverted cone getting deeper toward the cen- tral portion of the sub- strate (FIG. 7)
Today's most serious problems in Table 1 is unevenness of etching. Such etching unevenness becomes a serious problem especially in case of epitaxial wafers and SOI (silicon on insulator) wafers which need analysis of respective layers. A solution for increasing surface evenness of etching would be executing gentle etching by decreasing HF (hydrofluoric acid) concentration. However, this is not practical because it invites an increase of the amount of etching solution and causing the above-indicated problem, i.e., increase of the background level of impurities. Additionally, each technique optimizes the amount of the reagent it uses upon development thereof, and it is undesirable to change the amount of liquid also from the viewpoint of etching accuracy.
It would be also possible to decrease the temperature for reaction, taking account of etching being exothermic reaction, as means for increasing the surface evenness. However, it has been known to the Inventor through experiments that this retards the etching reaction itself and largely increases the time required. The Inventor further made a trial for uniform etching by continuously stirring the etching solution. However, this was also ineffective to improve the surface evenness, and stirring was rather liable to invite contamination from environments and could not overcome the problem.